1. Field of the Invention
The present invention relates to a fuel injector control system for an internal combustion engine of a cylinder injection type in which fuel is directly injected into cylinders of the engine. More specifically, the invention is concerned with a fuel injector control system capable of preventing or suppressing effectively generation of heat due to concentration of an electric current which may occur upon activation of high-speed fuel injection to the cylinders in a multi-cylinder internal combustion engine or upon divisional fuel injection over a plurality of strokes in an ordinary type internal combustion engine.
2. Description of Related Art
For having better understanding of the invention, background techniques thereof will first be reviewed. As the fuel injector control system for a cylinder injection type internal combustion engine in which there are employed a high-voltage generating means for overexcitation of the electromagnetic coil of the fuel injector at the beginning of the fuel injection and a low-voltage generating means for maintaining the injector in the open state, a fuel injector control system for a gasoline engine and a diesel engine have been well known in the art.
A typical one of such fuel injector control systems as type mentioned above is disclosed, for example, in Japanese Patent Publication No. 26701/1995. As is shown in FIG. 7 of this patent publication, there is generally adopted such arrangement in which a single high-voltage generating means is provided in common to all the fuel injectors to be controlled for realizing overexcitation thereof at the beginning of the fuel injection.
Additionally, a constant current source fed directly from an onboard battery of a motor vehicle is employed as a low-voltage generating means for supplying a constant current of low level to the fuel injector in order to hold the injector in the opened state in succession to the overexcitation thereof.
In this conjunction, it is noted that in the case of a multi-point injection type multi-cylinder engine including as many as six or eight cylinders and equipped with a corresponding number of fuel injectors separately for the individual cylinders, respectively, the time interval between the successive current supplies for the electric energization of the individual fuel injectors during a rotation of the engine is remarkably short when compared with the ordinary engine such as four-cylinder engine. Thus, the single high-voltage generating means provided in common to all the fuel injectors as mentioned above has to be so designed as being capable of accommodating a high current demand. To this end, the high-voltage generating means must be imparted with a large capacity or capability for the power supply, which is however difficult to implement practically. Besides, intensive current flow (i.e., concentration of the electric current) will occur the voltage generating circuit (including the high-voltage-generating means and the low-voltage generating means) upon every actuation thereof. In other words, an excessively large current may flow through the voltage generating circuit, which will result in generation of a large amount of heat. To say in another way, heat loss will increase to a great disadvantage from the viewpoint of economic or effective utilization of the electric energy.
Such concentration of the current and the intensive heat generation may be brought about or promoted equally by the low-voltage generating means as well.
In some ordinary type internal combustion engine having four or less cylinders, such a fuel injection system is adopted in which the fuel injection to each of the cylinders is performed a number of times over a plurality of strokes in order to ensure an adequate amount of fuel injection particularly during a high-speed operation of the engine. Such fuel injection for each cylinder over a plurality of strokes thereof is known as the divisional or distributive fuel injection.
More specifically, in the divisional or distributive fuel injection control system for the cylinder injection type internal combustion engine, the fuel injection is effected divisionally or distributively in both the suction stroke and the compression stroke in contrast to the conventional direct fuel injection system in which the fuel injection is validated only in the compression stroke. As a result of this, the fuel injection is performed simultaneously for both the cylinder which is in the compression stroke and the cylinder which is in the suction stroke (i.e., the cylinder for which the compression stroke is to follow) in the engine equipped with the divisional fuel injection system. Consequently, a heavy load will be imposed on the power supply circuit. For this reason, the divisional direct fuel injection system suffers a limitation in the response capability or performance for generation of a predetermined voltage required for effectuating the simultaneous divisional fuel injections for the two cylinders although such limitation depend on the capability or capacity of the high-voltage generating means and the low-voltage generating means.
At the beginning (or in the initial phase) of the electric energization of the fuel injector for actuation thereof, it is required to supply a large current to the electromagnetic coil of the fuel injector by applying a high voltage in order to ensure opening of the valve of the fuel injector with high reliability. In this conjunction, it is noted that when the high-voltage generating means is driven continuously throughout the overexcitation period, an increased amount of electric energy will be consumed ineffectively (i.e., useless power consumption will increase). Besides, electromagnetic noise is likely to be generated by a boost-up circuit employed for generating a high voltage, which noise will exert adverse influence to a control unit and peripheral devices installed on the engine and the motor vehicle.
As is apparent from the foregoing description, the conventional fuel injector control system for the cylinder injection type internal combustion engine known heretofore in which the voltage generating circuit (including the high-voltage generating means and the low-voltage generating means) for driving the individual fuel injectors designed for injecting the fuel directly into the associated cylinders, respectively, is realized as a single circuit suffers such problems difficult to solve in practical applications that the capability of the high-voltage generating means and the low-voltage generating means is insufficient for driving the injectors with high reliability and that the concentrative heat generation takes place in the voltage generating circuit particularly in the case of the multi-cylinder internal combustion engine in which the time interval between the electric energizations of the fuel injectors is short or in the ordinary type internal combustion engine in which the divisional fuel injection is adopted.
Another problem of the conventional fuel injector control system can be seen in that a large amount of electric energy tends to be consumed uselessly and that electromagnetic or radio noise is generated during the overexcitation period at the beginning of the electrical energization of the coil of the fuel injector.